1. Field of the Invention
This invention relates to a group-III nitride semiconductor light-emitting device comprising an n-type semiconducting layer and a p-type semiconducting layer formed of a group-III nitride semiconductor, between which is disposed an n-type light-emitting layer formed of a group-III nitride semiconductor containing indium.
2. Description of the Prior Art
In recent years, light-emitting diodes (LEDs) and laser diodes (LDs) that use a light-emitting layer formed of a group-III nitride semiconductor are being commercialized as light-emitting devices. As the group-III nitride semiconductor used to constitute the light-emitting layer, there is mainly used gallium indium nitride of composition formula: GaxIn1xe2x88x92xN (0xe2x89xa6xc3x97xe2x89xa61) in which the composition ratio of indium is adjusted to obtain the emission of the desired short wavelength visible light (see JP-B 55-3834).
The light-emitting section having a gallium indium nitride light-emitting layer can be broadly divided into three types, in terms of structure. The first type is a single heterojunction (SH) structure comprised of an n-type light-emitting layer and a p-type cladding layer; the second is a double heterojunction (DH) structure in which an n-type light-emitting layer is sandwiched between p-type and n-type barrier layers (cladding layers); and the third is a quantum well (QW) junction structure in which a well layer is n-type GaxIn1xe2x88x92xN.
In such structures, the basic mechanism by which light is emitted is radiation recombination of carriers, the carriers that are recombined being electrons and holes. This radiation recombination of carriers generates the radiation of light having a wavelength that corresponds to the difference xcex94E between the energy levels of the carrier electrons and holes. The relationship between the wavelength xcex (nm) of the light radiated by electron transition and the energy level difference xcex94E (eV) can be approximated by equation (1).
xcex=1.24xc3x97103/xcex94Exe2x80x83xe2x80x83(1)
A brief explanation will now be given of the mechanism by which radiation recombination gives rise to light emission. This mechanism is common to the three structures of the light-emitting section described above. Here, the explanation will be given with respect to the third type of structure, the quantum well.
FIG. 17 is a depiction of the ideal band arrangement of a quantum well structure. Depicted is a single quantum well (SQW) structure or one structural unit of a multi quantum well (MQW) structure.
In the figure, a potential well W1 and W2 is formed on the conduction band side and valence band side, respectively, of a well layer W. A potential barrier is formed at each side of the quantum well layer W by a band offset (discontinuity) with each barrier layer B in contact with the quantum well layer W. This band discontinuity arises from the bandgap difference of the nitride semiconductors that constitute the emission and barrier layers.
In the quantum well structure, quantized levels (quantum levels) L11 and L12, and L21 and L22 are formed within the potential well W1 and W2, as denoted in the drawings by the broken lines. The radiation recombination that takes place is produced by the transition of carriers between these quantum levels. That is, transitions are generated between electrons and light holes or between electrons and heavy holes, adhering to the wavelength selection rule in the k-space, whereby light is radiated having a wavelength that corresponds to the quantum level differential therebetween. In the case of FIG. 17, for example, transitions arise between electron quantum level L11 and hole quantum level L21, and in accordance with equation (1), this results in the radiation of light having a wavelength that corresponds to the energy level differential therebetween.
Also, the quantum levels L11, L12, L21 and L22 are distributed uniformly in the width direction (horizontal direction) of the quantum well layer W, into a rectangular distribution, while retaining a fixed potential differential from the conductance band.
In the light-emitting layer formed from the quantum well layer W in which the quantum levels L11, L12, L21 and L22 have a rectangular distribution, it is necessary to suppress variation in the wavelength of the emitted light in order to obtain full monochromaticity. To do this, it is necessary to have an abrupt change between the quantum well layer and barrier layer bands, and to be able to stably reproduce this abrupt band change. For this, it is important to achieve abrupt compositional changes with respect to the constituent elements in the junction interface between the quantum well layer and the barrier layer.
However, effecting this abrupt compositional change in the junction interface requires the use of a high level of interfacial control technology. In practice, the band change between the quantum well and barrier layers is gradual, as shown in FIG. 18.
With respect to FIG. 18 which shows a comparison between an actual band structure D1 and an ideal band structure D2, indicated by the broken line, when the band offset is gradual, quantum level L3 in the quantum well layer W decreases to L31 and quantum level L4 decreases to L41. This is the phenomenon that is generally recognized as quantum level fluctuation (see page 227 of fourth printing of first edition xe2x80x9cPhysics and Applications of Semiconductor Superlattices,xe2x80x9d edited by the Physical Society of Japan and first published on Sep. 30, 1986, by K. K. Baifukan, a Japanese corporation).
This quantum level fluctuation is also caused by slight changes in the thickness of the quantum well layer. With respect to FIG. 19, for example, if the thickness (width) of the quantum well layer W, as indicated by the solid line, is reduced to the thickness indicated by the broken line, quantum level L5 is elevated to L51. If the thickness is increased, as indicated by the dot-dash line, L5 decreases to L52. It is extremely difficult to control the thickness of the quantum well layer to a precise enough degree that ensures that there is no change in the quantum level.
Quantum level fluctuation has been held to cause instability of energy level differentials and variations in the wavelength of light emissions. Light-emitting devices having a quantum well structure with a conventional light-emitting layer of gallium indium nitride are prone to the effects of gradual compositional changes in the junction interface and slight changes in the thickness (width) of the quantum well layer W, changing emission wavelengths and therefore making it difficult to stably utilize the monochromatic properties.
In addition to the fact that light-emitting devices with a gallium indium nitride light-emitting layer are susceptible to fluctuations in quantum level, a further problem is that in such devices, it is difficult to form a good-quality junction interface.
This is because the qualities of the gallium indium nitride are readily altered by heating, producing a multiplicity of phases with varying indium concentrations (compositions), meaning there is a strong tendency to form a multi-phase structure (see (1) Solid State Commun., 11 (1972), pp 617 to 621; (2) J. Appl. Phys., 46 (8) (1975), pp 3432 to 3437).
FIG. 20 is a drawing showing the known structure of a light-emitting section that includes a light-emitting layer formed of gallium indium nitride. In the drawing, barrier layers B10 adjoining a well layer W10 are comprised of a binary (two-element) compound, gallium nitride (GaN), that enables phase changes to be ignored. The qualities of the gallium indium nitride mixed-crystal forming the light-emitting layer (well layer) W10 are readily altered by heat, producing a separation between the internal matrix phase P10 and the crystallite subsidiary phase P11. The matrix phase P10 and subsidiary phase P11 have different indium concentrations, and a correspondingly different bandgap. Variation also arises in the band offset based on the type of phase that is present in the junction interfaces between the light-emitting layer W10 and the barrier layers B10. Variations in the band offset will cause changes in the quantum level in the well layer W10. That is, in the case of a light-emitting section that includes a light-emitting layer formed of gallium indium nitride, in addition to fluctuations in quantum levels caused by the difficulty of controlling the composition at the junction interface and the layer thickness, quantized level fluctuation is also caused by the multi-phase tendency of the gallium indium nitride, which also results in a loss of monochromaticity.
Also with reference to FIG. 20, the light-emitting section comprised of gallium nitride/gallium indium nitride/gallium nitride is formed by forming the gallium nitride of the lower layer at 1100xc2x0 C., then lowering the formation temperature to a temperature range of from about 800xc2x0 C. to about 900xc2x0 C., forming the intermediate gallium indium nitride layer, then raising the temperature back up to over 1000xc2x0 C., to a temperature suitable for the formation of the gallium nitride upper layer. The heat produced when the temperature is increased to the temperature used to form the gallium nitride upper layer can cause sublimation of the gallium indium nitride intermediate layer, resulting in localized loss of the light-emitting layer. This localized loss of the light-emitting layer caused by sublimation and the like can change the thickness (well width) of parts of the light-emitting layer.
Thus, along with its multi-phase tendency, gallium indium nitride light-emitting layers are also susceptible to changes in the thickness (width) of the well layer arising in the course of the layer formation processes. These changes in layer thickness give rise to quantized level fluctuation, resulting in non- uniformity in the wavelength of the emitted light. The result is a loss of monochromaticity and a weakening of the intensity of the emitted light.
Thus, owing to the fact that (1) the difficulty of forming abrupt compositional ratios at the junction interfaces of the light-emitting section results in gradual band changes, (2) the wavelength of the emitted light is highly susceptible to slight changes in the thickness of the light-emitting layer, (3) the light-emitting layer tends to readily assume a multi-phase structure (non-miscibility), and (4) the light-emitting layer is prone to sublimation, while a light-emitting device in which the light-emitting layer is formed of gallium indium nitride might function, it will be subject to instability of the emitted light wavelength, loss of monochromaticity, and a relatively low emission intensity.
These problems produced a demand for a light-emitting arrangement capable of stably providing excellent carrier-recombination-induced (transitions between quantum levels) light emission properties. The present inventor therefore focused on a device in which carrier recombination is effected using a non-rectangular, rather than a rectangular, potential well arrangement.
A two-dimensional electron gas field effect transistor (TEGFET) is an example of a device in which carrier recombination takes place in a non-rectangular potential well structure. One type of TEGFET is the high mobility field effect transistor (modulation doped field effect transistor (MODFET)) formed of group-III-V compound semiconductor.
FIG. 21 shows the band structure of a MODFET. As shown, the MODFET band structure is non-rectangular in shape, and is comprised of an electron channeling layer (a channel layer) S1, a spacer layer S2 and an electron supplying layer S3. Conduction band and valence band discontinuities 501 and 502 are formed in a region inside the channel layer S1 in the vicinity of the junction interface 510 between the channel layer S1 and the spacer layer S2. There is a pronounced drop at the bend 503 in the conduction band, the conduction band level in the region in the vicinity of the junction interface 510 falling below the Fermi level F. Since this part of the conduction band that is below the Fermi level F has a low potential, electrons e accumulate and are localized there.
A comparison of electron (carrier) distribution states shows that quantized electrons in the conventional rectangular potential well layer are not localized at the junction interface with the barrier layer, but are instead substantially evenly distributed in the well layer at a level that keeps energy differential relative to the conduction band substantially constant. In contrast, electrons in the non-rectangular band structure of the MODFET accumulate and are localized in a region in the vicinity of the junction interface with other layers, that is, in a specific region of the channel layer S1 that constitutes the active layer.
In the MODFET, the electrons e are accumulated in the non-rectangular potential well formed at the bend of the conduction band structure, where they are confined two-dimensionally so as not to behave three-dimensionally to thereby utilize them as carriers for high-speed device operation.
There are GaAs/AlGaAs system MODFETs, GaInAs/AlGaAs system MODFETs. Recently, there are also gallium nitride system MODFETs (see, for example, Appl. Phys. Lett., 69 (25) (1996), pp 3872 to 3874). These gallium nitride MODFETs are formed of gallium nitride and aluminum gallium nitride mixed-crystal, with the gallium nitride being used to form the channel layer S1 and the aluminum gallium nitride mixed-crystal being used to form the spacer layer S2 and electron supplying layer S3 (see Appl. Phys. Lett., 69 (6) (1996), pp 794 to 796). It has already been reported that changes in band structure are produced that give rise to electron localization in a region in the vicinity of the junction interface between the gallium nitride and the aluminum gallium nitride mixed-crystal (Appl. Phys. Lett., 69 (23) (1996), pp 3456 to 3458).
Looking at the junction arrangement of the MODFET, the changes in band structure producing carrier (electron) localization are made at the junction interface between n-type conduction bands: that is, in the nn junction portions between n-type channel layer and n-type spacer layer or electron supplying layer. Looking at the order in which the crystal layers are formed, from the substrate crystal side, there is also what is generally referred to as an inverse-structure MODFET (see IEEE Electron Device Lett., EDL-7(1986), pp 71 and 454). In that case, the accumulation of carriers (electrons) as a two-dimensional electron gas is in the nn junction portions between n-type channel layer and n-type electron supplying layer or spacer layer. Thus, in each case MODFET carrier localization utilizes nn junction portions.
Concerning group-III nitride semiconductor devices that use a light-emitting layer of gallium indium nitride mixed-crystal, there are reports the contents of which suggest the feasibility of light emission via quanta in a confinement state in the light-emitting layer (see 28a-D-6 on page 178 of the Extended Abstructs No. 1, the 44th Meeting, The Japan Society of Applied Physics and Related Societies, 1997). However, the actual carrier confinement state is unclear, so the contribution that the carriers having what type of distribution in what type of band structure make to recombination giving rise to light emission is unknown.
Thus, as described above, a conventional light-emitting section with a gallium indium nitride light-emitting layer has a rectangular potential well structure and has problems that include a gradualized band change, susceptibility to the effect of differences in the thickness of the light-emitting layer, multi-phasing (non-miscibility) of the light-emitting layer and ready sublimation. Owing to these problems, light emitted by the light-emitting section is subject to variations in wavelength, loss of monochromaticity and loss of intensity.
On the other hand, while it has been found to be feasible to use a non-rectangular potential well structure to enable carrier recombination to be used to stably realize excellent light emission properties, the fact is that, in the field of light-emitting devices, the non-rectangular potential well structure has not been elucidated, and as such, neither the carrier distribution in such a structure, nor the component elements that contribute to the improvement of the light emission properties, are clearly known, and therefore it has not reached the stage where the light monochromaticity and emission intensity can be improved.
An object of the present invention is to provide a group-III nitride semiconductor light-emitting device capable of the high-intensity emission of short wavelength light having excellent monochromaticity from blue to green emission regions, by clarifying the optimum arrangement for applying a non-rectangular potential well structure to a light-emitting layer and effecting a major improvement in the light monochromaticity and emission intensity characteristics.
To attain the above object, the present invention provides a group-III nitride semiconductor light-emitting device comprising an n-type semiconductor layer formed of a group-III nitride semiconductor, a p-type semiconductor layer formed of a group-III nitride semiconductor, and an n-type light-emitting layer formed of an indium-containing group-III nitride semiconductor and disposed between the n-type semiconductor layer and the p-type semiconductor layer, wherein a thin layer of a group-III nitride semiconductor having a thickness of from not less than 3 nm to not more than 30 nm and a carrier concentration of not more than 5xc3x971017cmxe2x88x923 is disposed between the n-type light-emitting layer and the p-type semiconductor layer, the n-type light-emitting layer has a donor concentration of from not less than 5xc3x971017cmxe2x88x923 to not more than 1xc3x971019cmxe2x88x923, and an electron localized region is provided in a region of the n-type light-emitting layer in a vicinity of a junction interface with the thin layer in which are selectively accumulated and localized electrons having a sheet carrier concentration of from not less than 1xc3x971011cmxe2x88x922 to not more than 5xc3x971013cmxe2x88x922.
In this invention, as described above, an electron localized region is provided in a part of the n-type light-emitting layer that is in the vicinity of the junction interface with the thin layer, so that when, under a forward bias, holes of the p-type semiconductor layer diffuse via the thin layer into the light-emitting layer side of the junction interface, the holes flow down into the adjoining electron localized region and recombine with electrons in the electron localized region. Recombination therefore takes place smoothly and a high light-emission intensity can be obtained. The smoothness of the recombination also enhances high-speed emission on/off response characteristics. Also, the provision between the n-type light-emitting layer and the p-type semiconductor layer of a thin layer having a carrier concentration not exceeding 5xc3x971017cmxe2x88x923 and the provision of the electron localized region adjacent to the thin layer enables the conduction band offset on the thin layer side of the n-type light-emitting layer to be increased, thereby making it possible to deepen the conduction band bend at the electron localized region in the n-type light-emitting layer. Thus, electrons can be confined more securely in the electron localized region for more efficient recombination, resulting in better light emission characteristics.
Moreover, by giving the n-type light-emitting layer a donor concentration of from not less than 5xc3x971017cmxe2x88x923 to not more than 1xc3x971019cmxe2x88x923, the number of electrons that normally exhibit three-dimensional behavior in the three-dimensional space of the n-type light-emitting layer (three-dimensional carriers) is limited, making it possible to maintain the number of electrons that flow into the electron localized region at an appropriate level.
Also, by ensuring that in the electron localized region the sheet carrier concentration of electrons exhibiting mainly two-dimensional behavior (two-dimensional carriers) is not less than 1xc3x971011cmxe2x88x922 and not more than 5xc3x971013cmxe2x88x922, the proportion of the electron localized region taken up by two-dimensional carriers can be optimized with respect to the three-dimensional carriers in the three-dimensional space of the light-emitting layer.
Since the donor concentration of the n-type light-emitting layer and the sheet carrier concentration of the region of the n-type light-emitting layer in the vicinity of the junction interface with the thin layer are thus prescribed, the quantity of electrons in the electron localized region can be maintained at an appropriate level, thereby optimizing the light emission characteristics.
The device of the invention also includes one having a configuration in which the region of the n-type light-emitting layer in the vicinity of the junction interface with the thin layer is provided with a low carrier concentration layer having a carrier concentration that is lower than the carrier concentration in the region of the n-type light-emitting layer in the vicinity of the junction interface with the n-type semiconductor layer, and in which the low carrier concentration layer is provided with an electron localized region.
With the provision of a low carrier concentration layer in the region of the n-type light-emitting layer in the vicinity of the junction interface with the thin layer, and the provision of the low carrier concentration layer with an electron localized region, electrons from the surrounding light-emitting layer having a high carrier concentration can flow more easily into the electron localized region. Also, the electron localized region has a low concentration of impurities, meaning it is a region of high purity, so obstacles to the channeling of electrons caused by impurities do not readily arise, allowing high-speed, two-dimensional channeling of electrons to proceed smoothly.
The device of the present invention also includes one having a configuration in which the carrier concentration of the low carrier concentration layer is not more than 5xc3x971017cmxe2x88x923 and the layer thickness is from not less than 2 nm to not more than 20 nm. As a result, the movement of electrons that have accumulated in the electron localized region is restricted to a two-dimensional plane, controlling three-dimensional behavior and eliminating the wasted consumption of energy. Thus, since the electrons can actively move in the two-dimensional plane, recombination of electrons and holes can take place at higher speed, and it is possible to prevent reductions in the quantities of electrons accumulating in the electron localized region, and increases in the forward current.
The device of the invention also includes one having a configuration in which the n-type light-emitting layer is formed as a multi-phase structure comprised of matrix and subsidiary phases having different indium concentrations, and an n-type intermediate layer in which the conduction band discontinuity with respect to the matrix phase in the junction interface with the n-type light-emitting layer is not more than 0.2 eV is interposed between the n-type semiconductor layer and the n-type light-emitting layer.
By interposing an n-type intermediate layer between the n-type semiconductor layer and the n-type light-emitting layer in which the band offset on the conduction band side of the junction interface with the light-emitting layer is not more than 0.2 eV, the band offset between the intermediate layer and the light-emitting layer is decreased to the extent that the junction can be regarded as substantially a homojunction. Thus, this junction configuration serves to inhibit carriers (electrons) supplied from the n-type semiconductor layer side from staying in the junction interface on the n-type layer side of the light-emitting layer, thereby smoothly guiding the carriers into the light-emitting layer, and also has the effect of selectively and efficiently causing the carriers to be accumulated in the electron localized region provided in the light-emitting layer in the vicinity of the junction interface with the p-type semiconductor layer. These carriers efficiently undergo radiation recombination with the holes of the p-type semiconductor layer, increasing the light emission intensity with excellent monochromaticity.
The device of the invention also includes one having a configuration in which the intermediate layer is comprised of a group-III nitride semiconductor that on the side of the junction interface with the n-type light-emitting layer has a composition that is the same as, or close to, the matrix phase.
By forming the intermediate layer of a semiconductor that on the side of the junction interface with the light-emitting layer is the same as, or similar to, the matrix phase, what is substantially a homojunction between the intermediate layer and the light-emitting layer can be reliably and stably achieved, even when a light-emitting layer having a multi-phase structure is used.
The configuration may also include one in which the intermediate layer is comprised of an indium-containing group-III nitride semiconductor having a bandgap that gradually increases going toward the n-type semiconductor layer.
In such a configuration in which the intermediate layer is formed of an indium-containing group-III nitride semiconductor having a bandgap that gradually increases, going from the n-type light-emitting layer side toward the n-type semiconductor layer, the conduction band offset between the light-emitting layer and the n-type semiconductor layer can be increased smoothly in stages, going toward the n-type semiconductor layer from the n-type light-emitting layer. This therefore facilitates the channeling of carriers (electrons) from the n-type semiconductor layer side to the light-emitting layer, and contributes to the radiation recombination.